The present invention relates to electronic displays and, in particular, to reducing the rate of deterioration of display material in such displays.
Traditionally, electronic displays such as liquid crystal displays have been made by sandwiching an optoelectrically active material between two pieces of glass. In many cases each piece of glass has an etched, clear electrode structure formed using indium tin oxide. A first electrode structure controls all the segments of the display that may be addressed, that is, changed from one visual state to another. A second electrode, sometimes called a counter electrode, addresses all display segments as one large electrode, and is generally designed not to overlap any of the rear electrode wire connections that are not desired in the final image. Alternatively, the second electrode is also patterned to control specific segments of the displays.
Conventional liquid crystal displays are monostable, i.e., in the absence of any potential difference between the electrodes, the liquid crystal molecules assume random orientations, which renders the liquid crystal material non-transmissive of light, and indeed in such displays a given pixel is rendered non-transmissive simply by removing the potential difference between its associated electrode and the counter electrode, thereby allowing the molecules within this pixel to relax to random orientations. To maintain any given pixel in a transmissive state, it is necessary to drive the associated electrode substantially continuously.
Electrophoretic and other bistable displays have been the subject of intense research and development for a number of years. (The term xe2x80x9cbistablexe2x80x9d is used herein in its conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. The bistable characteristics of such displays are discussed in more detail below.) Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to cluster and settle, resulting in inadequate service-life for these displays.
An encapsulated, electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates.
In electrophoretic and other bistable displays, it has been commonly observed that the display fails after some time. One of the reasons why such a display may fail is that the materials used to construct the display are damaged by repeated application of electrical addressing signals. In particular, the application of a signal of one volt over a distance of one micron (one micrometer), or ten microns results in field strengths applied to the capsule of one million volts per meter or one hundred thousand volts per meter, respectively. These are quite large field strengths.
The present invention provides a solution that overcomes these and other problems that are encountered in conventional addressing methods that have been used in the prior art to address bistable displays. This invention provides novel methods and apparatus for controlling and addressing such displays. Additionally, the invention discloses applications of these methods and materials on flexible substrates, which are useful in large-area, low cost, or high-durability applications.
In one aspect, the present invention relates to a first method for addressing a bistable display element having first and second display states differing in at least one optical property. The method comprises (a) applying a first addressing signal to the display element that does not substantially change the display state of the display element; and (b) applying a second addressing signal to the display element that does change the display state of the display element.
In another aspect, the present invention relates to a second method for addressing a bistable display element having first and second display states differing in at least one optical property. The method comprises (a) applying to the display element a first addressing signal effective to change the display state of the display element; and (b) thereafter applying to the display element a second addressing signal that does not change the display state of the display element.
Embodiments of both these methods may have the following features. The display element can be an electrophoretic element, desirably an encapsulated electrophoretic display element. The electrophoretic display element may comprise an electrophoretic medium comprising a liquid and at least one particle disposed within this liquid and capable of moving therethrough on application of an electric field to the medium. Such an element may have a viewing surface and the liquid can have an optical property differing from that of the particle disposed therein so that the display element is in its first display state when the particle(s) lie(s) adjacent the viewing surface and in its second display state when the particle(s) is/are spaced from the viewing surface so that the liquid lies adjacent the viewing surface. Alternatively, the element may have a viewing surface and the liquid can have disposed therein at least one first particle having a first optical property and a first electrophoretic mobility and at least one second particle having a second optical property different from the first optical property and a second electrophoretic mobility different from the first electrophoretic mobility, so that the display element is in its first display state when the first particle(s) lie(s) adjacent the viewing surface and is in its second display state when the second particle(s) lie(s) adjacent the viewing surface.
The first method of the invention can include the step of applying to the display element a first addressing signal having a first polarity, a first amplitude as a function of time, and a first duration, such that the first addressing signal does not substantially change the optical property displayed by the display element. The first method can also include the step of applying to the display element a second addressing signal having a second polarity opposite the first polarity, a second amplitude as a function of time, and a second duration such that the second addressing signal substantially changes the optical property displayed by the display element.
The second method of the invention differs from the first in that, in the second method, the addressing signal which changes the state of the display element is applied first, and thereafter there is applied the addressing signal which does not change the state of the display element. While at first it might appear difficult if not impossible to apply a substantial second addressing signal which does not change the state of the display brought about by the first addressing signal, in practice bistable displays, especially electrophoretic bistable displays, typically exhibit a xe2x80x9cthresholdxe2x80x9d signal level, i.e., it is possible to apply a small signal to a display element without changing the display state of the element, even though a larger signal of the same polarity applied to the display element would bring about a rapid change of display state. Thus, the second method of the invention is conveniently practiced by making the first addressing signal of brief duration and large amplitude, while the second addressing signal is of lengthy duration but small amplitude. Typically, the second addressing signal has amplitude not greater than about one-fifth of the amplitude of the first addressing signal and a duration at least about five times the duration of the first addressing signal; preferably, the second addressing signal has an amplitude not greater than about one-tenth of the amplitude of the first addressing signal and a duration at least about ten times the duration of the first addressing signal.
In both the first and second methods of the present invention, desirably the sum of the first amplitude as a function of time integrated over the first duration and the second amplitude as a function of time integrated over the second duration is substantially zero, i.e., the algebraic sum of the integral of the first addressing signal with respect to time and the integral of the second addressing signal with respect to time is substantially zero. In a preferred embodiment, this algebraic sum is smaller in absolute magnitude than 10 Volt-seconds; in a more preferred embodiment, smaller in absolute magnitude than 1 Volt-second; and in a still more preferred embodiment, smaller in absolute magnitude than 0.1 Volt-seconds. The aforementioned sum (expressed in volt-seconds) is, in a preferred embodiment, smaller in absolute magnitude than one-tenth of the maximum amplitude expressed in volts of the larger of the first and second amplitudes; in a more preferred embodiment, this sum is smaller in absolute magnitude than one one-hundredth of this maximum amplitude; and in a still more preferred embodiment, this sum is smaller in absolute magnitude than one one-thousandth of this maximum amplitude.
The methods can include using first and second addressing signals of opposite polarity.
The methods can include steps of applying first and second addressing pulses such that the second amplitude differs from the first amplitude, or the second duration differs from the first duration. The methods can include steps of applying first and second addressing pulses such that the sum of the product of the first amplitude and the first duration and the product of the second amplitude and the second duration is substantially zero. The methods can also include using first and/or second addressing signals comprising a plurality of addressing pulses. The methods can include the step waiting for a predetermined period of time after step (a) and before step (b).
In another aspect, the present invention relates to a third method for addressing a bistable display, this display comprising a set of display elements each having first and second display states differing in at least one optical property. The method comprises: (a) selecting a first subset of display elements that represent a first image to be displayed, and applying to this first subset a first addressing signal, thereby causing first subset to assume their first display state and the electrophoretic display to display the first image; and (b) selecting a second subset of display elements that represent a second image to be displayed, thereby defining three classes of display elements, namely a first class which are members of both the first and second subsets, a second class which are members of the first subset but not members of the second subset, and a third class which are not members of the first subset but are members of the second subset, and applying to the second class a second addressing signal, thereby setting said second class to their second display state, and applying to the third class a third addressing signal, thereby setting said third class to their first display state and causing the display to display the second image.
In another aspect, the present invention relates to a fourth method for addressing a bistable display, this display comprising a set of display elements each having first and second display states differing in at least one optical property. The method comprises (a) causing the display to display a first image in which a first subset of display elements are in their first display state and the complement of the first subset of display elements are in their second display state; (b) applying to the first subset of display elements, but not to the complement of the first subset, a first addressing signal, thereby causing the first subset to assume their second display state; and (c) applying to a second subset of display elements, different from said first subset, but not to the complement of the second subset, a second addressing signal, thereby causing the second subset to assume their first display state and the display to display a second image, different from the first image, the second image being formed by the second subset of display elements being in their first display state and the complement of the second subset being in their second display state.
Embodiments of both these methods may have the following features. The display element can be an electrophoretic element, desirably an encapsulated electrophoretic display element. The electrophoretic display element may comprise an electrophoretic medium comprising a liquid and at least one particle disposed within this liquid and capable of moving therethrough on application of an electric field to the medium. Such an element may have a viewing surface and the liquid can have an optical property differing from that of the particle disposed therein so that the display element is in its first display state when the particle(s) lie(s) adjacent the viewing surface and in its second display state when the particle(s) is/are spaced from the viewing surface so that the liquid lies adjacent the viewing surface. Alternatively, the element may have a viewing surface and the liquid can have disposed therein at least one first particle having a first optical property and a first electrophoretic mobility and at least one second particle having a second optical property different from the first optical property and a second electrophoretic mobility different from the first electrophoretic mobility, so that the display element is in its first display state when the first particle(s) lie(s) adjacent the viewing surface and is in its second display state when the second particle(s) lie(s) adjacent the viewing surface.
The methods can include, before step (a), the step of applying to all the display elements of the set a blanking signal sufficient to cause every display element of the display to display its second display state. The method may further comprise the step of applying to all the display elements, prior to the application of the blanking signal thereto, a pre-blanking signal sufficient to cause every display element of the display to assume its first display state. If such a blanking signal is used in the fourth method of the invention, it is necessary, before step (a), to apply to the first subset of display elements, but not to the complement of the first subset, a third addressing signal, thereby causing the first subset of display elements to assume their first display state.
In both the third and fourth methods of the invention, desirably the sum of the integral of the amplitude of the first addressing signal as a function of time over the duration of the first addressing signal plus the integral of the amplitude of the second addressing signal as a function of time over the duration of the second addressing signal is substantially zero, i.e., the algebraic sum of the integral of the first addressing signal with respect to time and the integral of the second addressing signal with respect to time is substantially zero. When both a pre-blanking and a blanking signal are applied prior to step (a), it is also desirable that the algebraic sum of the integral of the pre-blanking signal with respect to time and the integral of the blanking signal with respect to time be substantially zero. In a preferred embodiment, this algebraic sum is smaller in absolute magnitude than 10 Volt-seconds; in a more preferred embodiment, smaller in absolute magnitude than 1 Volt-second; and in a still more preferred embodiment, smaller in absolute magnitude than 0.1 Volt-seconds. The methods can include the step of waiting for a pre-determined period of time after step (a). Also, as in the first and second methods described above, in the third and fourth methods of the invention either or both of the first and second addressing signals may comprise a plurality of addressing pulses.
The second and fourth methods of the invention previously described may be used to address a display having on one side thereof a common electrode extending across all the display elements of the set, and on the opposed side thereof a plurality of discrete electrodes, one of the discrete electrodes being associated with each of the display elements of the set. When the fourth method of the invention is used to address such a display, desirably the first addressing signal is provided by setting the common electrode to a first voltage and the discrete electrodes associated with the first subset of display elements to a second voltage different from the first voltage, while the second addressing signal is provided by setting the common electrode to the second voltage and the discrete electrodes associated with the second subset of display elements to the first voltage. This form of the fourth method of the invention may use the blanking and pre-blanking signals previously described, and if so the pre-blanking signal is conveniently provided by setting the common electrode to the second voltage and all the discrete electrodes to the first voltage, while the blanking signal is conveniently provided by setting the common electrode to the first voltage and all the discrete electrodes to the second voltage.
In yet another aspect, the present invention provides a fifth method of addressing a bistable display, this display comprising a set of display elements each having first and second display states differing in at least one optical property. This fifth method comprises (a) selecting a first subset of display elements that represent a first image, and applying to the first subset a first addressing signal, thereby causing the first subset to assume their first display state and the display to display the first image; (b) selecting a second subset of display elements that represent a second image different from the first image and thereby defining four classes of display elements, namely a first class which are members of both the first and second subsets, a second class which are members of the first subset but not members of the second subset, a third class which are not members of the first subset but are members of the second subset, and a fourth class which are not members of either the first or the second subset, and applying to the second class a second addressing signal, thereby setting the second class to their second display state, and applying to the third class a third addressing signal, thereby setting the third class to their first display state, and causing the display to display the second image. In this fifth method, the display has on one side thereof a common electrode extending across all the display elements of the set, and on the opposed side thereof a plurality of discrete electrodes, one of the discrete electrodes being associated with each display element of the set, and the common electrode is kept at a substantially constant first voltage during steps (a) and (b).
In a preferred form of this fifth method, the discrete electrodes associated with at least one (and preferably both) of the first and fourth classes of display elements are kept at substantially the constant first voltage during steps (a) and (b). Also, desirable the second addressing signal is provided by setting the discrete electrodes associated with the second class of display elements to a second voltage substantially equal to the first voltage plus a predetermined difference, and the third addressing signal is provided by setting the discrete electrodes associated with the third class of display elements to a third voltage substantially equal to the first voltage minus the predetermined difference. Also, desirably the second and third addressing signals are each preceded by a pre-addressing signal, this pre-addressing signal being provided by setting the discrete electrodes associated with the second class of display elements to substantially the third voltage and the discrete electrodes associated with the third class of display elements to substantially the second voltage.
In yet another aspect, the present invention provides a sixth method of addressing a bistable display, comprising a set of display elements each having first and second display states differing in at least one optical property, the display having on one side thereof a common electrode extending across all the display elements of the set, and on the opposed side thereof a plurality of discrete electrodes, one of the discrete electrodes being associated with each display element of the set. The sixth method comprises (a) selecting a first subset of display elements that represent a first image; (b) selecting a second subset of display elements that represent a second image different from the first image and thereby defining four classes of display elements, namely a first class which are members of both the first and second subsets, a second class which are members of the first subset but not members of the second subset, a third class which are not members of the first subset but are members of the second subset, and a fourth class which are not members of either the first or the second subset, (c) bringing the display to a state in which the first and classes of display elements are in their first display state and the third and fourth classes are in their second display state, thereby causing the first image to be displayed; and (d) applying a first voltage to the common electrode for a first period and a different second voltage to the common electrode for a second period, while maintaining the discrete electrodes associated with at least one of the first and fourth classes of display elements at substantially the same voltages as the common electrode, and maintaining the discrete electrodes associated with one of the second and third classes of display elements substantially at the first voltage while maintaining the discrete electrodes associated with the other of the second and third classes substantially at the second voltage, thereby causing the first and third classes of display elements to be in their first display state and the second and fourth classes of display elements to be in their second display state, so that the display displays the second image. In a preferred form of this sixth method, in step (d) the discrete electrodes associated with both the first and fourth classes, of display elements are maintained at substantially the same voltage as the common electrode. Desirably, the first and second periods are substantially equal in duration.
In an especially preferred form of this sixth method, the common electrode is subjected to a plurality of cycles each comprising a first period in which the first voltage is applied and a second period in which the second voltage is applied, and the discrete electrodes associated with one of the second and third classes of display elements are maintained substantially at the first voltage and the discrete electrodes associated with the other of the second and third classes are maintained substantially at the second voltage throughout the plurality of cycles.
In yet another aspect, the present invention relates to a bistable display element having first and second display states differing in at least one optical property, and a signal control module that controls the signal applied to the display element, the signal control module applying at least a first addressing signal and at least a second addressing signal to the display element, the first addressing pulse not substantially changing the display state of the element and the second addressing signal changing the display state of the element. This signal control module is designed to carry out the first method of the invention. This invention also provides a similar bistable display element in association with a signal module arranged to apply at least first and second addressing signals to the display element, the first addressing pulse changing the display state of the element and the second addressing signal not substantially changing the display state of the element. This signal control module is designed to carry out the second method of the invention.
Embodiments of these aspects of the invention have the following features. The display element can be an electrophoretic display element, desirably an encapsulated electrophoretic display element. The electrophoretic display element may comprise a liquid and at least one particle disposed within this liquid and capable of moving therethrough on application of an electric field to the medium. Such an element may have a viewing surface and the liquid can have an optical property differing from that of the particle disposed therein so that the display element is in its first display state when the particle(s) lie(s) adjacent the viewing surface and in its second display state when the particle(s) is/are spaced from the viewing surface so that the liquid lies adjacent the viewing surface. Alternatively, the element may have a viewing surface and the liquid can have disposed therein at least one first particle having a first optical property and a first electrophoretic mobility and at least one second particle having a second optical property different from the first optical property and a second electrophoretic mobility different from the first electrophoretic mobility, so that the display element is in its first display state when the first particle(s) lie(s) adjacent the viewing surface and is in its second display state when the second particle(s) lie(s) adjacent the viewing surface.
The signal control module can apply a first addressing signal that has a first polarity and a second addressing signal that has a second polarity opposite the first polarity. Either or both of the first and second addressing signals can comprise a plurality of addressing pulses. The first addressing signal can have a first polarity, a first amplitude as a function of time and a first duration, and the second addressing signal can have a second polarity, a second amplitude as a function of time and a second duration, such that the sum of the first amplitude as a function of time integrated over the first duration and the second amplitude as a function of time integrated over the second duration is substantially zero.